1. Field of the invention:
This invention relates to an apparatus for binarizing image signals, and more particularly to an apparatus for converting image signals generated from a television camera or the like into binary signals.
2. Description of the prior art:
FIG. 7 shows a conventional apparatus of such a kind. The apparatus of FIG. 7 comprises an A/D converter 55, a density distribution memory 56, and a control unit 57. The image signals of an object 53 and a background 53a obtained by a television camera 54 are supplied to the A/D converter 55, and converted into digital signals. The digital image signals are stored in the density distribution memory 56. The control unit 57 determines a threshold level for binarizing the object 53 (hereinafter, such a threshold level is referred to as "a binary level"), from the density distribution of the image signals as described below.
FIG. 8 shows one example of the density distribution of the image signals stored in the memory 56. As shown in FIG. 8, the density distribution of the image signals obtained from the object 53 and background 53a has two peaks or maximal values 58 and 59 and one valley or minimal value 60. The control unit 57 judges the minimal value 60 sandwiched between the two maximal values 58 and 59 as the binary level, and then converts the image signals into binary signals by using the thus-determined binary level.
When the density of the object 53 is not uniform, however, the density distribution will have two or more valleys as shown in FIG. 9. In such a case, it is impossible to determine the binary level uniquely. When there is little difference in density between the object 53 and the background 53a, moreover, the contour of the image may be obscure, with the result that the contour of the obtained binary image become unstable.